Janessa and Nick Whatcott, of Bluffdale, Utah, weren’t too worried when their 6-year-old son Nixon began complaining of leg pain in January.

“He’s really active, so my husband and I thought maybe he’d sprained his ankle or had a hairline fracture,” Janessa says of Nixon. “He’d be like, ‘Ow, my knee hurts.’ There were a couple times where it hurt really bad … but the next day he’d be off running and playing.”

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Nixon continued to complain of the pain, and after about three weeks Janessa and Nick took the boy to a local hospital. There, Janessa says, she expected doctors to confirm her suspicions that Nixon was simply experiencing “growing pains.” However, the doctor’s visit did not go as the couple planned.

“Our doctor … took a couple X-rays then he came into the room and asked us to come out with him,” she says, noting that the doctor then showed the couple Nixon’s shocking results.

“We went back and he pointed it out and said, ‘We found a tumor in him on his bone.’ ”

Nixon (left) with mother Janessa Whatcott

Courtesy Janessa Whatcott

The X-ray showed a tumor the size of an egg on Nixon’s left femur. The doctor said they’d need to determine whether the tumor was cancerous.

“I was horrified. That’s every parent’s worst nightmare,” Janessa tells PEOPLE. “I was in total belief that it was growing pains, and that I was being overly worried. It was scary to figure that out and have to accept what that was.”

The family went to a hospital in Salt Lake City where doctors determined that the tumor was cancerous —and found several smaller tumors also on the boy’s left femur.

“My husband and I lost it. We felt that loss that he’d been given this thing and we couldn’t take it away from him. There was nothing we could do for him. We were concerned what the cancer treatments would be. How were we gonna tell him?” the mom of four says.

Whatcott family

Courtesy Janessa Whatcott

“We were devastated for him. He’s such a happy go-lucky little boy and you’d never even think that he was sick. The day before, he was just running outside with his brothers. Looking at him, he didn’t look sick. It was almost unbelievable. We were just devastated. We couldn’t wrap our heads around it.”

Over the next few days, the family learned that the cancer had spread to Nixon’s lungs and his arm. He has spent most of the months since in the hospital, and has undergone six rounds of chemotherapy. Janessa and Nick share health updates on a Facebook page called NixonStrong. Photos on the page show Nixon and his siblings sporting wide smiles.

But Janessa says Nixon’s illness has taken a toll on the family.

Nixon Whatcott

Courtesy Janessa Whatcott

“It’s really rocked the whole family. It’s been hard for [my sons] to not have me there in the mornings to take them to school or pick them up from school,” she says. “It’s hard for Nixon because Nixon misses his brothers. He misses playing with them, going to school and just being a kid. It affects every single person in the family in a different way.”

Now, Janessa says the chemotherapy has helped and the boy’s health is improving.

Nixon Whatcott

Courtesy Janessa Whatcott

Soon, he’ll have to undergo a rotationplasty, in which surgeons will remove his cancerous femur and knee. Nixon will then get a prosthetic which will function as his knee.

“Nixon’s been frustrated having to come to the hospital a lot. It was hard for him because he wants to have a real leg, he doesn’t want to have a fake leg,” Janessa tells PEOPLE. “This has turned our lives upside down, but [Nick and I] just keep in mind that we only have to do this for a short time and he’ll be healthy.”